Connected: An Internet Encyclopedia
RFC 1771 - 9.1 Decision Process

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### 9.1 Decision Process

The Decision Process selects routes for subsequent advertisement by applying the policies in the local Policy Information Base (PIB) to the routes stored in its Adj-RIB-In. The output of the Decision Process is the set of routes that will be advertised to all peers; the selected routes will be stored in the local speaker's Adj-RIB- Out.

The selection process is formalized by defining a function that takes the attribute of a given route as an argument and returns a non- negative integer denoting the degree of preference for the route. The function that calculates the degree of preference for a given route shall not use as its inputs any of the following: the existence of other routes, the non-existence of other routes, or the path attributes of other routes. Route selection then consists of individual application of the degree of preference function to each feasible route, followed by the choice of the one with the highest degree of preference.

The Decision Process operates on routes contained in each Adj-RIB-In, and is responsible for:

- selection of routes to be advertised to BGP speakers located in the local speaker's autonomous system

- selection of routes to be advertised to BGP speakers located in neighboring autonomous systems

- route aggregation and route information reduction

The Decision Process takes place in three distinct phases, each triggered by a different event:

1. Phase 1 is responsible for calculating the degree of preference for each route received from a BGP speaker located in a neighboring autonomous system, and for advertising to the other BGP speakers in the local autonomous system the routes that have the highest degree of preference for each distinct destination.

2. Phase 2 is invoked on completion of phase 1. It is responsible for choosing the best route out of all those available for each distinct destination, and for installing each chosen route into the appropriate Loc-RIB.

3. Phase 3 is invoked after the Loc-RIB has been modified. It is responsible for disseminating routes in the Loc-RIB to each peer located in a neighboring autonomous system, according to the policies contained in the PIB. Route aggregation and information reduction can optionally be performed within this phase.

9.1.1 Phase 1: Calculation of Degree of Preference
9.1.2 Phase 2: Route Selection

9.1.2.1 Breaking Ties (Phase 2)

9.1.3 Phase 3: Route Dissemination
9.1.4 Overlapping Routes

Next: 9.1.1 Phase 1: Calculation of Degree of Preference

Connected: An Internet Encyclopedia
RFC 1771 - 9.1 Decision Process